Previous high reliability high voltage (&gt;100 V) space qualified fuses have been gas-filled. Because gas-filled fuses may leak in a low pressure (&lt;1 Torr) or vacuum environment such as space, the inside of the fuse may reach critical pressure. A fuse is said to have blown when the fuse element melts or vaporizes, thus interrupting the current through the fuse. If the fuse is blown when the internal pressure is at critical pressure, the fuse will not interrupt the current. The current will instead continue to flow as a flaming arc, destroying the fuse package and surrounding materials. FIG. 1 shows a current and voltage record of this event. At time T=1 second, the voltage increased, but even though the fuse blew at 2 seconds, the current continued to flow in an electric arc. The arc burned until the voltage was removed. This performance was demonstrated repeatedly with 10 A, 125 v fast acting mini-fuses which were evacuated through a 5-10 mil hole in the tubular fuse body.
A liquid or solid immersion of the fuse element may avoid the critical pressure problem if the liquid or solid immersion may be achieved and maintained during the melting and explosive current interruption of the fuse element. Suppliers of fuses have recognized the need for cheap board mounted, small fuses and have developed devices with solid, non gas potting to fill this need. However, presently available fuses are limited to a 6 ampere, 125 volt rating. Above 6 amperes, the voltage rating is reduced to lower the peak burnout energy.
Small amounts of insulating silicone have been utilized in micro components for the purpose of absorbing shock or stress from thermal expansion, providing thermal insulation, and protecting surrounding material. However, large quantities of silicone have not been used to encapsulate the fuse element and suppress arcing.
U.S. Pat. No. 4,935,848 discloses a partially-covered fuse element with an elastic silicone resin for the purpose of preventing a short circuit to an adjoining wall of a capacitor. The thickness of the silicone used does not suggest that the silicone is used to suppress an arc occurring when the fuse blows. The patent also teaches that a plurality of bubbles should be mixed with the silicone, a feature that would inhibit any arc suppressing characteristic the silicone might possess. The present invention encases the fuse element in a thick layer of silicone which is substantially free of air bubbles. The silicone suppresses any arc which may be formed when the fuse blows.
U.S. Pat. No. 4,720,772 similarly describes use of a silicone resin in a micro-fuse and suggests the formation of air bubbles within the resin. It teaches that the bubbles increase the thermal insulating properties of the elastic resin and thus facilitate the melting of the fuse element. The present invention teaches away from forming air bubbles and uses silicone for its elastic properties.
U.S. Pat. No. 4,763,228 describes use of a thin silicone layer with a thickness on the order of the fuse element diameter to prevent charring of the surrounding resinous encapsulant. The silicone layer used in the present invention must be substantially thicker in order to elastically flow into the space left by the blown fuse.
U.S. Pat. No. 4,169,271 describes the use of a small droplet of silicone as a material with a small coefficient of thermal expansion that will not carbonize when heated. When surrounded by a material with a high coefficient of thermal expansion, the silicone is compressed and tends to pinch off the liquefied metal wire before sufficient heat builds up to ignite surrounding material. The present invention covers the entire fuse element with silicone and is intended to operate at higher voltage and current.
U.S. Pat. No. 4,814,946 also describes use of a small strip of silicone to prevent carbonization in a low power device. Arc suppression is not mentioned.
U.S. Pat. No. 5,027,101 describes use of a silicone layer to insulate a sub-miniature (&lt;0.1") fuse, but does not encapsulate the fuse element. The present invention completely encapsulates the fuse element and is used at higher power.
U.S. Pat. No. 4,873,506 discloses arc quenching techniques for fractional and low ampere fuses. It uses a metallo-organic thin film as the fuse element. The present invention uses a metal wire as fuse element and operates at higher power.
A Buss SMD Tron surface mount fuse specification dated February 1991 discloses the use of fuses in high power applications. However, the present invention is operable at higher voltage and current levels. Observation of the material which surrounds the Buss fuse element shows that it does not possess the elastic characteristic of RTV silicone. The elasticity of the silicone in the present invention enables the silicone to snap back into the gap formed by the blown element and quench the arc.
An undated Mepcopal specification discloses the use of a glass coating as an arc suppressant.